"People are not approaching it on the basis of the no bodybags rule," a defence source said.

Afghanistan will be extremely hazardous for US and British special forces because they will be up against battle-hardened warriors familiar with the mountains and passes.

The head of German special services, Brigadier General Reinhard Günzel, whose troops are expected to go to Afghanistan, warned that any attempt to capture Bin Laden would lead to a "bloodbath". He accepted that, because of the scale of the atrocities in New York and Washington, the soldiers were going to have to try anyway.

Washington and London are anxious about the impact on public morale of lots of bodybags being brought home from a war that they have predicted will be lengthy.

The British government yesterday began preparing public opinion. The Northern Ireland secretary, John Reid, told the BBC's Breakfast with Frost programme that people had to be ready for greater sacrifices than suffered in Kosovo: "I think we've got to expect that. And we have to be prepared to tolerate that."

He added: "We shouldn't avoid the proposition that there will be danger to us. We are America's biggest ally. Obviously in standing up to terrorism there is the danger of sacrifice of lives _ [but] to do anything other is essentially to capitulate to terrorism."

Brig Günzel told the German magazine Der Spiegel he considered the task of capturing Bin Laden to be "just about impossible" and said this view was shared by special forces in the US, Britain, Israel and France. But he added the Washington attacks had probably changed the willingness of politicians to accept risks.

Although Germany is still wavering on whether its troops should be involved, the German media have reported a small force of its elite troops was about to be sent on a secret mission to central Asia. Four of the eight aid workers detained in Afghanistan since early August on charges of promoting Christianity are German.

The Bush administration has already begun preparing public opinion for the coming conflict.

It has gone out of its way to warn American citizens that this will be a military campaign like no other and that they should be prepared for sacrifices.

In his televised speech to both houses of Congress last Thursday, Mr Bush said: "This war will not be like the war against Iraq a decade ago, with a decisive liberation of territory and a swift conclusion. It will not look like the air war above Kosovo two years ago, where no ground troops were used and not a single American was lost in combat."